Fluid or powder fluid including plural kinds of solid components may be represented by river water, seawater, and blood. Each of these is mixture of liquid component and solid component. The component, such as sand, bacteria and blood cells, is precipitated or dispersed in solid form, and does not melt in the liquid component.
A method of separating such components, for example, for separating blood cells from blood will be described. Blood is usually collected as whole blood that includes blood plasma (liquid component), blood cells (solid component), and other components. However, only the blood cells or only blood plasma is often required for a blood test.
In order to inspect, for example, a blood-sugar level in blood, the amount of the blood sugar melting in the blood plasma is measured. In order to detect DNA, the DNA is extracted from leukocyte of blood cells. For this purpose, in a conventional method, in order to separate the collected whole blood into blood plasma and blood cells, the whole blood is put into a test tube, and then, a predetermined centrifugal force is applied to the blood with a centrifugal separator. This operation allows components of the whole blood in the test tube to receive different strength of centrifugal forces according to respective masses of the components, hence causing the components to be separated.
Then, the blood plasma is taken out by extracting supernatant liquid, and so the blood cells are taken from precipitation. Then, the separated component is measured in predetermined properties in a process of the test.
The conventional method employing the centrifugal separator has the following problem. The method requires a certain amount, e.g. several dozen milliliters, of whole blood in the test tube. Hence, it is difficult to separate solid components from liquid if the amount of sample is insufficient.
A method of separating solid components from a small amount of sample with a filter is disclosed in “A novel fabrication of In-channel 3-D micromesh structure using maskless multi-angle exposure and its microfilter application” (Hironobu Sato, MEMS2003, Kyoto, pp. 223-226, published by IEEE) and “Integrated vertical screen microfilter system using inclined SU-8 structure” (Yong-Kyu Yoon, MEMS2003, Kyoto, pp. 227-230, published by IEEE). In this method, a filter with porousness filtrates blood cells larger than a predetermined size to separate blood cells from a blood plasma. The size and the number of holes of the filter influences the characteristic of separation. Therefore, the filter needs to be optimally designed according to component to be separated. For example, a mesh-like filter with a desirable size can be manufactured by exposing photosensitive resist to light three-dimensionally, and the filter can have predetermined number and size of holes accurately.
In the conventional method employing a filter, the particle size of solid components varies according to a pressure for having fluid or powder fluid with plural components pass through the filter. Particularly, solid component particles having sizes different from each other can be hardly separated by this method. In order to take predetermined particles, a hole size of the filter is determined so that only particles smaller than the size pass through the filter. However, the predetermined particles are trapped by the filter, and clog the holes of the filter. Hence, the particles may prevent smaller particles from passing through the filter.